1. Field
This disclosure relates to antennas for satellite communications earth stations.
2. Description of the Related Art
Satellite communications systems use one or more orbiting satellite to relay communications between a pair of earth stations. Each earth station typically consists of a transmitter and a receiver coupled to a highly directional antenna. A common form of antenna for transmitting to and receiving from a satellite consists of a parabolic dish reflector and a feed network. Given the large distance between each earth station and the satellite, each earth station must be configured to transmit a relatively high power signal and to receive a very low power signal. To ensure that transmission from a first earth station does not interfere with reception at a second proximate earth station, earth station antennas must be designed to have very low side lobe and back lobe radiation.
Earth station antennas typically have either a center-feed or an offset-feed. In a typical center-feed antenna, the feed network is located along the axis of the parabolic reflector, and thus blocks a portion of the reflector aperture. In an offset-feed antenna, the reflector is an off-axis portion of a parabolic dish and the feed network is located to one side where it does not block a portion of the reflector aperture. Center feeds are commonly used with large diameter reflectors, since the feed network may block only a negligible portion of the reflector aperture. Offset feeds are commonly used with small reflectors where a center feed network would block a substantial portion of the reflector aperture.
Since the feed network of an offset-feed antenna is located to the side of the reflector, an offset-feed antenna occupies a larger volume than a center-feed antenna for equivalent reflector aperture. In some applications, such as portable or mobile earth stations, an antenna may be mounted on a gimbal configured to point the antenna at any desired angle within a hemisphere. In this case, an offset-feed antenna will sweep a substantially larger volume than a center-feed antenna of equivalent aperture, and thus require a substantially larger radome.
In this patent, the term “circular waveguide” means a waveguide segment having a circular cross-sectional shape. Similarly, the term “annular waveguide” means a waveguide segment having a cross-sectional shape of an annulus between two concentric circles. In this patent, the term “port” refers generally to an interface between devices or between a device and free space. A port of a waveguide device may be formed by an aperture in an interfacial surface to allow microwave radiation to enter or exit a waveguide within the device.
Elements in the drawings are assigned three-digit reference numbers where the most significant digit indicates the figure number where the element was introduced. An element not described in conjunction with a figure may be presumed to be the same as a previously-described element having the same reference number.